At least five people have died as a result of heavy weather in the state of Oklahoma after a tornado alert was issued across the Midwest of the US.

Dozens of tornadoes swept through parts of Oklahoma, Kansas, Nebraska and Iowa overnight.

The five deaths were reported when a twister struck the northwest city of Woodward early on Sunday morning after lightning apparently disabled its storm warning system, Mayor Roscoe Hill said.

Two children died at the Hide A Way mobile home park on the west side of Woodward, a town of 12,000 people, while two adults were killed in a small community just outside the city limits, Hill said.

Almost unprecedented

Details of the fifth death were not immediately known, according to Keli Cain, a spokesperson for Oklahoma's emergency management department.

The National Weather Service said in an advisory on Saturday afternoon that severe storms were possible from Texas in the south to Iowa, South Dakota and Minnesota in the north.

"It does look like we have the potential for a massive outbreak of tornadoes throughout the remainder of the day," our meteorologist said.

"[All the factors] make it look like it's the right combination for more storms to come."

Local Oklahoma City television station KOCO reported the twister in Woodward caused "significant structural damage and several injuries".

In the Iowa town of Creston, the Greater Regional Medical Center hospital was damaged by a possible tornado, said a woman who answered the phone there but declined to give her name.

A dispatcher for the Union County sheriff's office said he was unable to release any information about damage to the hospital or buildings.

Severe hail

In northwestern Oklahoma, a tornado touched down for less than a minute in the afternoon, according to Rick Smith, a National Weather Service meteorologist in the town of Norman.

The area was, however, hit by severe hail, breaking windows and damaging buildings.

A tornado was also spotted in Mustang, a suburb of Oklahoma City, before dawn. Kristy Yager, a spokeswoman for Oklahoma City, said that trees, power lines and fences were reported to have collapsed in the area.

"We'll have storms all night long in Oklahoma," Smith said. The pair of Oklahoma twisters hit a day after a tornado sliced across Norman on Friday afternoon.

The Federal Emergency Management Agency said the worst conditions were expected to hit late on Saturday between Oklahoma City and Salina, Kansas, while other areas could see baseball-sized hail and strong winds.

The US tornado season started early this year, with twisters already blamed for 57 deaths in 2012 in the Midwest and South, raising concerns that this year would be a repeat of 2011, the deadliest tornado year in nearly a century.